Gaseous substances are analyzed and continuously monitored in a large number of applications, such as environmental analysis, control of chemical processes, monitoring of production facilities, and in both civil and military fields for the detection of chemical warfare agents (CWAs) or explosives. Ion mobility spectrometry is a method introduced in the 1970s for high-sensitivity detection of potentially dangerous analyte substances in ambient air or other sample gases.
An ion mobility spectrometer (IMS) can be operated at ambient pressure. Ion mobility spectrometers are relatively compact and can be manufactured simply and inexpensively, making them particularly suitable for portable and mobile gas monitoring and warning devices.
Analyte substances to be detected are ionized in a reaction chamber of an ion mobility spectrometer. The ions move in a drift gas under the influence of an electrical field and are separated according to their different drift velocities, as a function of their mobility. Alternatively, in FAIMS, the dependency of their mobility on the field strength is used. If the parameters of the drift gas, such as its temperature, humidity and pressure, are held constant, then the mobility parameters of the ions are characteristic of the various substances. In the majority of mobile ion mobility spectrometers, the drift gas is circulated in a closed internal gas circuit by a pumping device, such as a gas pump or a fan. A filter incorporated into the internal gas circuit removes moisture from the circulating gas and purifies it of the analyte substances before returning the gas to the measuring tube of the ion mobility spectrometer. The parameters of the circulating gas are maintained as constant as possible inside the closed internal gas circuit. U.S. Pat. No. 4,311,669 to Spangler et al. discloses a method where the analyte substances generally enter the closed internal gas circuit of the ion mobility spectrometer through a permeable inlet membrane, on which sample gas impinges from the outside. Typically the membrane is made of silicone rubber. The majority of analyte substances permeate through the membrane relatively easier than water, thereby reducing the unfavorable ingress of moisture.
U.S. Pat. No. 5,083,019 to Spangler describes a method for increasing the detection power of an ion mobility spectrometer, by coupling the ion mobility spectrometer to a substance collector or preconcentrator. A collecting tube or a surface made of a material that adsorbs or dissolves the analyte substances to be detected, is exposed to a sample gas during a collecting phase. The analyte substances are largely accumulated in the substance collector. In a subsequent desorption phase, the collected analyte substances are released, usually by heating the substance collector. If the volume of gas that flows through the substance collector during the desorption phase is significantly lower than in the collecting phase, the concentration increases correspondingly. As a result, the detection power of the ion mobility spectrometer is increased. However, storage and delays slow penetration effects during the passage through the membrane inlet, resulting in the broadening of short analyte substance pulses released at the substance collector over time. The maximum concentration in the interior of the measuring tube is inversely proportional to the temporal broadening of the analyte substance pulse. The inverse relationship lowers the enrichment factor, in particular for slowly penetrating analyte substances of low volatility, which are particularly difficult to detect in practice. To obtain a high detection power, a correspondingly large quantity of analyte substance must be collected, which in practice causes long collection times and therefore long delay times for warnings, or high suction flows and large substance collectors, which are expensive and consume relatively large amounts of energy.
Beside a high detection power, a short warning time is an essential feature of an ion mobility spectrometer. However, disadvantageously in the aforementioned prior art, the warning time varies between a few seconds and a few minutes.
In other prior art examples, German Publications DE 199 38 392 to Leonhardt et al. and DE 10 2005 004 325 to Landgraf disclose methods in which substances are collected in substance collectors outside the internal gas circuit. The substance collectors are subsequently incorporated into the internal closed gas circuit by valves changing the gas flow circuitry.